Researchers control reactions between just two atoms

Reactions depend on something called the "hyperfine state."

The types of chemical reactions we typically learn in school take place on a Cecil B. deMille, cast-of-thousands scale. They involve huge numbers of atoms or molecules, making it extremely difficult to study exactly what is happening on the quantum level between the individual players. Many of them won't even take place at cold temperatures, where the rate of reaction is slow enough to observe the details. All this makes it very difficult to understand the quantum mechanics of chemical reactions between individual pairs of atoms.

In a new Nature Physics paper, researchers with the Cavendish Laboratory at the University of Cambridge were able to measure the chemical interactions between individual, ultracold ytterbium ions and rubidium atoms. Lothar Ratschbacher, Christoph Zipkes, Carlo Sias, and Michael Köhl studied inelastic collisions between the atoms and ions, in which some of the energy in the system is converted to light or motion. In doing so, they obtained the first low-level analysis of charge exchange, the chemical reaction in which an electron is passed between an atom and an ion.

Ordinary atoms are electrically neutral: they possess equal numbers of protons and electrons. However, when two atoms approach each other slowly (as they do at cold temperatures) something odd happens: the electrons end up repelling each other, preventing chemical interactions.

This doesn't happen with ions—atoms in which one or more electrons are added or subtracted, giving them a net electric charge. When an atom and a positive ion approach slowly, the ion's charge draws part of the atom's electron cloud towards it, leading to an attractive force between them. This force is much weaker than if both atoms were ionized, but at cold temperatures, the two objects can approach each other slowly enough that it becomes significant.

The authors of the new study exploited this property by magnetically trapping ytterbium ions (Yb+) and neutral rubidium atoms (Rb) at very low temperatures. They set the Yb+ to one of two quantum states by exciting them with laser light; these states were chosen because they take a long time to decay to their ground state. This means the ions won't give up this extra energy before they interact with the Rb atoms.

Being in an excited state meant the ions had internal energy—energy unrelated to the motion of the ions, which is expressed as temperature. This is akin to macroscopic objects such as a rolling ball: in addition to the energy of its motion, it has internal energy in the form of the motion of the atoms inside the balls. The internal energy of the Yb+ ions is available during the chemical reactions.

In the experiment, the reactions were all exothermic. This means the additional internal energy from the excited state could be converted to kinetic energy, so that the products of the reaction moved faster after than they did before. (Alternately, the extra energy could be converted to photons—an example of fluorescence.)

In some cases, the kinetic energy was sufficient to kick the reaction products out of the trap entirely; knowing the amount of energy required to do this set a minimum bound on the final kinetic energy. If the products remained inside the apparatus, their final speed was measured, revealing how much energy had been transferred.

The researchers performed the experiment under two conditions. First, the reactions were run in the dark, with the laser shut off after being used to prepare the ions. No photons were present in the trap other than any that might have been emitted by the reaction itself. Second, they kept the laser directed onto the atom and ion, using it to control their interaction. By adjusting the frequency of the laser, the authors altered both the rate of the reaction and the quantum states of the products.

In some trials, one electron was transferred from the Rb atom to the Yb+ ion, so that the rubidium became ionized (Rb+) and the ytterbium became neutral. This reaction is known as charge exchange. In no cases did a molecule form from the reacting objects, though the experimentalists considered that to be a possible outcome.

One interesting thing the researchers noted: they found the relative orientation of the electronic spin and the nuclear spin—known as the hyperfine state—made a difference to the reaction outcome. The properties of an atomic nucleus typically don't play a direct role in chemical reactions, but these results show a clear counterexample to that general assumption.

Between the charge exchange, quenching, hyperfine state, and other properties, this experiment provides an excellent demonstration of how manipulation of quantum states can lead to chemical reactions at very low temperatures. By using single atoms in the reactions, the researchers created a particularly clean experimental environment, avoiding the complications that arise from using the usual large numbers of reacting objects.

21 Reader Comments

Awesome. I did something in Physics at Uni that is in some way interesting in the real world - Hyperfine spectra. Whooo ho! BTW you do not need that much kit. Just a basic light spectrometer and a mercury lamp.

Awesome. I did something in Physics at Uni that is in some way interesting in the real world - Hyperfine spectra. Whooo ho! BTW you do not need that much kit. Just a basic light spectrometer and a mercury lamp.

The types of chemical reactions we typically learn in school take place on a Cecil B. deMille, cast-of-thousands scale.

Well, had I looked at it that way, those 5 semesters of chemistry(nothing says sado-masochism like Organic Chemistry I && II) en route to an almost useless bio degree probably would have been more fun if nothing else.

Ordinary atoms are electrically neutral: they possess equal numbers of protons and electrons. However, when two atoms approach each other slowly (as they do at cold temperatures) something odd happens: the electrons end up repelling each other, preventing chemical interactions.

This isn't new or I totally misunderstood the quote. There are already labels warning us some electronic equipments are not operable under extreme hot or extreme cold environment. It's best at 50 to 90 degree F?

Thats what electrons do, I'm not sure what's so odd about it. e-e repulsion is a big component of reaction barriers.

For some reason I can't access the article through uni so I don't know if they discuss this, but I'm not too surprised about a correlation between hyperfine and rate. Hyperfine coupling is related to the electronic distribution (its not a pure nuclear property, but related to the interaction between the unpaired electrons and nuclear spin), so different hyperfine=different wave function=different rate.

Ordinary atoms are electrically neutral: they possess equal numbers of protons and electrons. However, when two atoms approach each other slowly (as they do at cold temperatures) something odd happens: the electrons end up repelling each other, preventing chemical interactions.

This isn't new or I totally misunderstood the quote. There are already labels warning us some electronic equipments are not operable under extreme hot or extreme cold environment. It's best at 50 to 90 degree F?

I think we are talking about entirely different definitions of cold here. These experiments probably had atoms cooled to less than i Kelvin. Normal chemical reaction at higher temperatures (guessing at temperature, but I would expect 60 Kelvin to be more than enough) happen so fast enough that the the electrons repulsive force become irrelevant. Or at least that is how I understood the article

Ordinary atoms are electrically neutral: they possess equal numbers of protons and electrons. However, when two atoms approach each other slowly (as they do at cold temperatures) something odd happens: the electrons end up repelling each other, preventing chemical interactions.

This isn't new or I totally misunderstood the quote. There are already labels warning us some electronic equipments are not operable under extreme hot or extreme cold environment. It's best at 50 to 90 degree F?

You're right its not new, it was just laying some background. I think you're thinking of the semiconductors in transistors. Too much heat and they have too many carriers (and too cold = not enough) to perform their function, which is why your computer needs cooling.

Some ions like to interactThis is a fact that I think you knowAt least we'd expect them to, (if they're not too slow)But if you freeze some YtterbiumThat's not what you'll findIf it's in a hyperfine state of mind

Simply mix with RubidiumIn your living room, or your physics labThen you may see it pinch a shell, (like a hermit crab)Just make sure your ions Are carefully confinedIf they're in a hyperfine state of mind

You may think it fantasticIf inelastic collisions occurReactions are exothermic, I think you'll concurNow wait with the lights offFor flouresence unrefinedIf it's in a hyperfine state of mind

A hyperfine state is akin to what you do at youth parties - first you start to catch up with the party, when you can decide how many beers (energy) will get you into an appropriate state (available energy levels).

Quote:

This is akin to macroscopic objects such as a rolling ball: in addition to the energy of its motion, it has internal energy in the form of the motion of the atoms inside the balls.

Feels like an unfinished analogy.

To take the ball and roll with it: imagine a read hot iron ball thrown into a room temperature sand pit. The motion energy (linear and rotational) would go into heating the sand, but so would the internal heat energy.

You could finetune the ball heating to make any effect dominate.

arswhat wrote:

But what does that mean for me? Will I get a flying car?

It is well known that research is an extraordinary good investment, but also that you can never predict in what form the payback comes.

Here we can do a more intelligent guess. This is allowing for more knowledge on basic chemistry.

And specifically interesting to me [astrobiology] is that individual, albeit parallel, reactions is what distinguish many cellular reactions. In a crowded cytoplasmic environment there is for instance only genes at specific places on the DNA molecules. These individual genes are read off, on hundreds of DNA sites in parallel, by individual molecular machines that assemble and disassemble based on chemical cues.

So this could be a way to get to know the biochemistry regime of cells.

I sort of have the same question, but to put it another way: I understand that this experiment gives researchers a clean environment where they can monitor the reaction between two atoms. But, what are the other experiments that can benefit from this and what potential real-world uses do they have?

Perhaps the answer is just, "we don't know just yet". Thought I would ask anyway.

Oh, another thought: Could this be used to better understand the reactions of atoms to better understand how to produce nuclear fusion?

This isn't new or I totally misunderstood the quote. There are already labels warning us some electronic equipments are not operable under extreme hot or extreme cold environment. It's best at 50 to 90 degree F?

Electronic devices don't undergo chemical reactions (not if they want to keep working, anyway). The temperature specs are the range in which the manufacturer guarantees* correct and reliable operation. Outside of that range, transistors might switch too fast or slow and analog/mixed signal components like memories and ADCs might produce errors. High temperatures also cause faster degradation. Non-solid state devices like LCDs might break down or stop working.

*The manufacturer really only guarantees that a certain fraction of the units will work. For consumer devices, this might approach 99.9%. Higher reliability (on the order of 1-10 defective parts per million) usually requires an agreement with the manufacturer, which is why spec sheets tells you to call before putting their product into a life support system.

Outside of that range, transistors might switch too fast or slow and analog/mixed signal components like memories and ADCs might produce errors. High temperatures also cause faster degradation. Non-solid state devices like LCDs might break down or stop working.

To add to the list, a major stumble block is that silicon bipolar transistors eventually suffer thermal runaway. The EC resistance drops as it heats, allowing more current so more power so more heat, et cetera.

You may think that doesn't affect modern circuits which are mostly CMOS, but a) they often have bipolar structures as protection between the pads and the more sensitive CMOS circuitry, b) some may have parasitic bipolars (i.e. unwanted transistor action) and c) some high performance circuits can be in BiCMOS technology for speed. So in order to not burn your circuit board, literary, keep its temperature within limits!